1. Field of the Invention
The present invention relates to an electrostatic image developer used for developing an electrostatic image in an electrophotographic method, electrostatic recording method and the like. The present invention also relates to an image formation method and image forming device using the electrostatic developer.
2. Description of the Related Art
In recent, an electrophotographic method for visualizing image information through an electrostatic latent image has been used in various fields. This method is well known as disclosed in U.S. Pat. Nos. 2,297,691 and 2,357,809. The electrophotographic method generally comprises a charging and exposure step of forming an electrostatic latent image on a photosensitive member, a developing step of developing the electrostatic latent image using a developer including a toner to form a toner image, a transfer step of transferring the toner image to a transfer member, e.g., paper and a sheet, a fixing step of fixing the toner image to the transfer member using heat, a solvent, pressure and the like to prepare a permanent image.
Although the toner used in the above electrophotographic method is designed to meet the requirements in the above steps, the toner is suffered from the stress and impact generated due to the contact with a charge donating member or other members in a developing unit to be deteriorated in its structure and properties during copying repeatedly. Therefore, the image is also deteriorated during copying repeatedly. The toner is required to be produced using a tough binder which can withstand a mechanical stress and impact, in order to provide a highly reliable image over a long period of time.
On the other hand, for process adaptability in a copying machine, the toner is required to have excellent fluidity, caking resistance, fixing capability, chargeability, cleaning characteristics and the like. In recent years, it has been proposed that additives with high add value which can provide excellent fluidity, caking resistance, charge maintenance capability, environmental stability, and the like were added to the toner. For example, it has been proposed that inorganic oxides such as silica, titanium oxides, or the like are externally added to the toner.
Silica has excellent caking resistance and ability to impart fluidity, but has the drawback of great environmental dependency of charging due to a core with highly negative chargeability. Therefore, there are various proposes in which the surface of silica is hydrophobically treated (see Japanese Patent Application Laid-Open (JP-A) Nos. 46-5782, 48-47345, 48-47346, 64-73354, and 1-237561). However, no methods including these proposals can sufficiently achieve the improvement in the environmental dependency of charging without adversely affecting the caking resistance, fluidity and the like of silica.
The external addition of titanium oxide to the toner leads to a low charging level of the toner. The titanium oxide in the same kind of core can easily control the charging level and the environmental dependency by using a surface-hydrophobic treating agent, while the titanium oxide has a problem of aggregation and the like after hydrophobic treatment, and limits the amount to be hydrophobically treated. Therefore, the titanium oxide cannot attain the charging level lower than the given level.
Titanium oxide can be generally produced by refining TiO(OH).sub.2 prepared in a sulfuric acid process (a wet process) using ilmenite ore followed by heating to sinter. Such a titanium oxide produced in this manner naturally include aggregated particles produced as a result of the dehydration-condensation.
However, it is not easy to redisperse such aggregated particles in conventional methods. Crystal type titanium oxide (rutile: a specific gravity of 4.2, anatase: a specific gravity of 3.9) obtained as micropowder forms secondary or tertiary aggregation and is inferior to silica in an ability to improve the fluidity characteristics of the toner. Especially, there is a tendency to use a smaller particle diameter, in order to meet the needs of the market for an excellent image. Use of the smaller particle diameter of the toner however leads to the increased adhesive force between particles, and thus the decreased fluidity of the toner and more decreased ability for titanium oxide to impart fluidity.
In order to achieve both of the improved fluidity and environmental dependency of charging, there is a proposal in which hydrophobic titanium oxide processed by surface hydrophobic treatment, in combination with hydrophobic silica is externally added to the toner (see JP-A No. 60-136755 and the like).
However, stress, e.g., stirring tends to induce a defect of either silica or titanium oxide so that it is difficult for the both to compensate the defects each other over a long period of time. For example, hydrophobic titanium oxide can primarily improve the chargeability and fluidity of the toner. A coupling agent is peeled off the surface by the collision between the toner and a carrier or the friction between the toner and a blade or a sleeve in stirring, and thus the charging properties of the toner greatly change. The inventors supposed that in case of hydrophobic titanium oxide, the reactivity of its core surface with the coupling agent is poor and thus the bonding with the coupling agent is much weaker than that of silica. There is also the problem that these external additives are embedded into the surface of the toner, which causes the decreased power fluidity, and which causes contamination of these external additives to the surface of a carrier.
On the other hand, it has been proposed to add externally a hydrophobic amorphous titanium oxide to the toner (see JP-A Nos. 5-204183 and 5-72797).
This method, however, has the problem that the adhesion of the amorphous titanium oxide with a photosensitive material is strong, which causes damages to the photosensitive material at the time of cleaning and the occurrence of white spots on images.
To the developer was added carrier particles generally called "carrier" as well as the toner. When the carrier is contained in the developer, a charge is generated by the friction between the toner and the carrier to provide the toner with an appropriate amount of positive and negative charge. The carrier is roughly classified into a coated type carrier having a surface coating layer and a non-coated type having no surface coating layer. Since the coated carrier is superior in terms of the life of the developer, various coated carrier have been developed and put to the practical use.
The coated carrier is required at least to provide the toner with adequate chargeability (charge amount and charge distribution) and to maintain the adequate chargeability over a long period of time.
Various coated carriers, which do not change the chargeability of the toner, have excellent impact resistance, frictional resistance and the like, and are stable to environmental changes, e.g., temperature and humidity, are proposed.
For example, JP-A Nos. 61-80161, 61-80162, and 61-80163 disclose that a copolymer of nitrogen-containing fluorinated alkyl(meth)acrylate and a vinyl-type monomer, or a copolymer of fluorinated alkyl(meth)acrylate and a nitrogen-containing monomer is applied to the surface of a carrier core to obtain a relatively long-lived coated carrier. Furthermore, there are disclosed that a polyamide resin (JP-A No. 1-118150) or a melamine resin (JP-A No. 2-79862) is applied to the surface of a carrier core and cured to obtain a coated carrier having a relatively hard coating.
These carriers, however, have the problems that the contamination of the toner to the carrier surface, i.e., the impaction cannot be perfectly prevented.
In order to prevent the impaction, for example, a silicone resin as described in JP-A No. 60-186844 or a fluorine-type resin as described in JP-A No. 64-13560 may be used to form a coating layer for the carrier.
However, in such a carrier, the silicone resin or the fluorine-type resin exists in a considerably large amount in the vicinity of the surface but in a little amount in the center. When such a carrier is used for a long period of time, a coating layer of the carrier is worn thereby gradually losing the effects of the silicone resin or the fluorine-type resin, while the impaction gradually occurs.
Therefore, the effects of each independent development of the external additives and the carrier are still insufficient to improve the developer.
A method in which toners are developed and transferred every color, specifically, developing and transfer steps are plurally repeated to form at least three toner layers on a same support member, e.g., paper, is used for full-color copying machines which attract considerable attention. However, the original image density is larger and the area coating between the toner and the photosensitive material is larger, as compared with the black-and-white development. The toner is therefore required to have excellent transfer characteristics. In addition, it is necessary that the color toner image fixed to an OHP film has excellent transparency. Since the transparency decreases with increase of the external additive, the toner may possess the function to prevent the significantly reduced transparency even in added external additives.